Two-wire signal transmitter and method for maintaining integrity of electrical contacts

Abstract

Two-wire signal transmitter and method in which a very low-level Alternating Circuit (AC) input signal sensed at a remote circuit at a first end of a signal path is added to a low-level Direct Current (DC) bias signal provided by a local circuit that is located at a second end of the signal path. In a preferred embodiment, the remote circuit is electrically coupled to and powered by the local circuit through a two-wire path interconnection, typically a shielded twisted-pair cable. The DC bias signal has sufficient level to overcome minimum ratings of electrical contacts, thereby to pass the sensed very low-level AC input signal reliably through the poor electrical contacts that are used, for instance, for connecting electronic circuits that run the length of a railway train. In a preferred embodiment, the use of a unity gain current amplifier as a part of the remote circuit reduces the signal degrading effects of long signal paths, but does not amplify the AC input signal, thereby retaining known pre-determined railroad vitality requirements. An output signal is provided that is an AC coupled, isolated, non-amplified representation of the AC input signal. An alternate preferred embodiment utilizes no active amplifier as a part of the remote circuit, thereby providing a signal transmitter and method that is less complex, has lower cost, requires smaller packaging, and has somewhat better long-term reliability than the preferred embodiment utilizing the active amplifier in the remote circuit.

What is claimed is: 
     
       1. A signal transmitter for transmitting an alternating current (AC) signal through an electrical contact, wherein the electrical contact has a minimum voltage rating and a minimum current rating that respectively exceeds a voltage level and a current level of the AC signal, the signal transmitter comprising: remote circuit means for sensing the AC signal;   a direct current (DC) power source;   local circuit means for biasing the AC signal with said DC power source;   interconnection means having solely two conductive paths for electrically interconnecting said remote circuit means with said local circuit means;   wherein a biased AC signal is formed by adding a DC signal bias that is provided by said DC power source to said sensed AC signal at said remote circuit means, thereby to exceed the minimum voltage and current rating of the electrical contact; and   wherein said DC signal bias is removed from said sensed AC signal at said local circuit means after being transmitted through the electrical contact via said interconnection means.   
     
     
       2. The signal transmitter according to claim 1, wherein said biased AC signal is transmitted from said remote circuit means to said local circuit means through the electrical contact and said interconnecting means. 
     
     
       3. The signal transmitter according to claim 2, wherein said DC power source powers both of said remote and local circuit means from said local circuit means and through said interconnecting means. 
     
     
       4. The signal transmitter according to claim 2, wherein said DC power source powers both of said remote and local circuit means from said remote circuit means and through said interconnecting means. 
     
     
       5. The signal transmitter according to claim 1, wherein said remote circuit means further comprises: a pick-up coil system for inductively receiving the AC signal.   
     
     
       6. A signal transmitter for transmitting an alternating current (AC) signal through an electrical contact, wherein the electrical contact has a minimum voltage rating and a minimum current rating that respectively exceeds a voltage level and a current level of the AC signal, the signal transmitter comprising: remote circuit means for sensing the AC signal, said remote circuit means comprising current amplifying means for amplifying the AC signal with zero DC bias, wherein said current amplifying means has unity gain, thereby to retain pre-determined railroad vitality requirements;   a direct current (DC) power source;   local circuit means for biasing the AC signal with said DC power source;   interconnection means having solely two conductive paths for electrically interconnecting said remote circuit means with said local circuit means;   wherein a biased AC signal is formed by adding a DC signal bias that is provided by said DC power source to said sensed AC signal at said remote circuit means, thereby to exceed the minimum voltage and current rating of the electrical contact; and   wherein said DC signal bias is removed from said sensed AC signal at said local circuit means after being transmitted through the electrical contact via said interconnection means.   
     
     
       7. The signal transmitter according to claim 6, wherein said interconnection means further comprises a cable having a first and a second electrical conduction path. 
     
     
       8. The signal transmitter according to claim 7, wherein said cable is a shielded twisted-pair cable. 
     
     
       9. The signal transmitter according to claim 6, wherein said local circuit means further comprises: inductance means for filtering said DC power source, wherein said inductance means places an apparent high impedance between said DC power source and said biased AC signal that has been transmitted from said remote circuit means to said local circuit means.   
     
     
       10. A method of transmitting an alternating current (AC) signal from a remote location to a local location through an electrical contact, wherein the electrical contact has a minimum voltage rating and a minimum current rating that respectively exceeds a voltage level and a current level of the AC signal, wherein the method comprises the steps of: a) sensing the AC signal from a rail of a railroad track;   b) amplifying the sensed AC signal with an amplifier having unity gain, thereby to retain pre-determined railroad vitality;   c) transmitting a DC bias signal through interconnection means for interconnecting the remote location and the local location through the electrical contact;   d) offsetting the AC signal with the DC bias signal such that the voltage level and the current level of the AC signal, when offset with the DC bias signal, respectively exceeds the minimum voltage rating and the minimum current rating of the electrical contact;   e) transmitting the AC signal that is offset with the DC bias signal to the local location through the electrical contact and said interconnection means;   f) removing the DC bias signal from the AC signal at the local location to provide an output AC signal with zero DC bias; and   g) outputting said output AC signal with zero DC bias at the local location as a representation of the sensed AC signal that was sensed at the remote location.   
     
     
       11. An electronic circuit for transmitting an alternating current (AC) signal through an electrical contact, wherein the electrical contact has a minimum voltage rating and a minimum current rating that respectively exceeds a voltage level and a current level of the AC signal, the electronic circuit comprising: a remote circuit that senses the AC signal;   a local circuit that outputs the AC signal;   a cable having solely two conductive paths for electrically interconnecting said remote circuit with said local circuit at the electrical contact;   a direct current (DC) power source that powers said remote circuit and said local circuit when said cable electrically interconnects said remote and local circuits;   wherein said DC power source provides a DC signal bias that is added to said sensed AC signal at said remote circuit, thereby to exceed the minimum voltage and current rating of the electrical contact; and   wherein said DC signal bias is removed from said sensed AC signal at said local circuit after being transmitted through the electrical contact via said cable.   
     
     
       12. The electronic circuit according to claim 11, wherein said remote circuit means further comprises: a pick-up coil system for inductively receiving the AC signal.   
     
     
       13. An electronic circuit for transmitting an alternating current (AC) signal through an electrical contact, wherein the electrical contact has a minimum voltage rating and a minimum current rating that respectively exceeds a voltage level and a current level of the AC signal, the electronic circuit comprising: a remote circuit that senses the AC signal, said remote circuit comprising current amplifying means for amplifying the AC signal with zero DC bias, wherein said current amplifying means has unity gain, thereby to retain pre-determined railroad vitality requirements;   a local circuit that outputs the AC signal;   a cable having solely two conductive paths for electrically interconnecting said remote circuit with said local circuit at the electrical contact;   a direct current (DC) power source that powers said remote circuit and said local circuit when said cable electrically interconnects said remote and local circuits;   wherein said DC power source provides a DC signal bias that is added to said sensed AC signal at said remote circuit, thereby to exceed the minimum voltage and current rating of the electrical contact; and   wherein said DC signal bias is removed from said sensed AC signal at said local circuit after being transmitted through the electrical contact via said cable.